A roller mill of the aforementioned kind is well-known from the patent literature. The Japanese patent application no. 2002370044 A2 discloses a vertical roller mill for grinding particulate material comprising a mill housing surrounding a rotatable grinding table and rollers which are configured for interactive operation with the grinding table. The roller mill has a main gas supply duct connected to the mill housing beneath the grinding table for introducing gases into the mill housing through a nozzle ring enclosing the grinding table. It is necessary to keep a high velocity over the nozzle ring to avoid material from falling by gravity through the nozzle ring down to a duct below the grinding table. The high circulation factor of the material over the nozzle ring, and the high gas velocity through the nozzle ring creates a high pressure loss across the nozzle ring, and thereby a high fan energy consumption as compared to the mill motor power consumption. In addition to the high energy consumption, the high gas velocity also causes substantial wear of the parts in the roller mill swept over by the gas. In the Japanese patent application a bypass duct which receives gases from the main supply gas duct is connected to the mill housing at a single location at a point above the grinding table. The gas flow from the bypass duct is increased when more gas is needed in the mill housing depending on the load in the mill. Both the gas flow through the bypass duct and the main supply duct are activated by means of the same fan and the flow in the bypass duct is regulated by a throttle valve. Thus it will not be possible to reduce the pressure loss since the pressure in the mill housing must be kept sufficiently low to maintain the velocity in the nozzle ring, and as a consequence hereof the amount of gas introduced through the bypass duct must be controlled by means of the mentioned throttle valve creating the same pressure loss as across the nozzle ring. Furthermore the bypass duct cannot singly make up for the introduction of the complete amount of needed gases in the roller mill since it only introduces gases at a single location in the mill housing. The main gas supply must be introduced at several locations around the grinding table in order to obtain a properly gas/material suspension.